Method of applying a powder coating to a length of a lignocellulosic material

ABSTRACT

A method of applying a powder coating to a length of a lignocellulosic material. The method comprises impregnating the length of lignocellulosic material with an impregnating composition (i) a dicarboxylic anhydride or a tricarboxylic anhydride dissolved in non-aqueous solvent; (ii) an isocyanate thermosetting resin dissolved in a non-aqueous solvent; or (iii) a combination of (i) or (ii). Any excess impregnating composition is then removed from the lignocellulosic material. Non-aqueous solvent is also removed and the impregnated lignocellulosic material is placed in either an electrostatic field or in a fluidized bed and a powder coating composition is applied thereto so that the powder coating composition adheres to the lignocellulosic material. The impregnated and coated lignocellulosic material is then subjected to elevated temperatures to polymerise and/or cross-link the resin and cure the powder coating composition to form a powder coating. The length of lignocellulosic material may be a sheet of paper, wood or wood veneer. A typical solvent is dichloromethane or liquid carbon dioxide.

This application is the national phase of international applicationPCT/GB97/01464 filed May 29, 1997 which designated the U.S.

BACKGROUND OF THE INVENTION

This invention relates to a method of applying a powder coating to alength of a lignocellulosic material, such as for example a sheet ofpaper.

Powder coating is the term given to the application of a decorativecoating principally to metallic articles. The coating is applied to thearticle in an electrostatic field by propelling dry pigmented particlesfrom a special gun, which is friction or electrostatically activated,towards the article, the particles being attracted to the article byelectrostatic forces. The particles adhere to the surface of thearticle, and depending upon the force of the electrostatic field,successive particles adhere until the required build up is achieved,whereafter any surplus powder falls from the article and may berecovered. The article is then moved through a suitable oven at elevatedtemperatures, usually In the range of 140° C., to 185° C., or at lowertemperatures in the presence of ultra violet light, to cause the powderparticles to melt, flow, coalesce and cure lo form a coating.

The advantages of powder coating are that a wide variety of textures andsurface finishes may be achieved, the coatings are very tough andresistant to wear and in exterior grades, resistant to weathering. Inaddition, the powder coating method is solventless and because thepowder can be recovered for reuse, wastage is virtually nil. Thethickness of the coating on the article may be very accuratelycontrolled. Further, the method is of particular application to articlesof complex shape. Powder coatings are further characterised by theirflexibility and adhesion so that, after powder coating, an article suchas a flat sheet may be post formed over curves or edges.

One powder coating technique requires that the article to be coated mustbe able to sustain an electrostatic field for the particles of thepowder coating composition to adhere thereto. It is possible that anarticle which does not retain an electrostatic field could be dampenedor wetted in order for the particles of the powder coating compositionto adhere to the article. However, oven heating of the article may leadto the commencement of decomposition, or to "blowing" as gasses escapetrough the coalescing powder film from the heated articles. Thealternative is fusion coating wherein the article is preheated beforeapplying the powder coating, such as in a fluidized bed.

There is thus a need for a method by which articles which normallycannot be powder coated, can have a powder coating applied thereto,

SUMMARY OF THE INVENTION

According to the invention there is provided a method of applying apowder coating to a length of a lignocellulosic material, which methodincludes the steps of:

(a) impregnating the length of lignocellulosic material with animpregnating composition comprising either:

(i) a dicarboxylic anhydride or a tricarboxylic anhydride dissolved in asuitable non-aqueous solvent; or

(ii) an isocyanate thermosetting resin dissolved in a suitablenon-aqueous solvent; or

(iii) a combination of a dicarboxylic anhydride or a tricarboxylicanhydride and an isocyanate thermosetting resin dissolved in a suitablenon-aqueous solvent;

(b) if necessary removing from the impregnated length of lignocellulosicmaterial any excess of the impregnating composition;

(c) removing the non-aqueous solvent or solvents;

(d) placing the impregnated length of lignocellulosic material in anelectrostatic field Or in a fluidized bed and applying a powder coatingcomposition thereto so that the powder coating composition adheresthereto; and

(e) then subjecting the length of lignocellulosic material to elevatedtemperatures to polymerise and/or cross-link the resin or resins in thelength of lignocellulosic material and to cure the powder coatingcomposition to form the powder coating.

The length of a lignocellulosic material may be for example a sheet ofpaper, A length of peeled or sliced wood veneer, a length of laminatedwood, chip board, fibre board, or the like.

DESCRIPTION OF EMBODIMENTS

The crux of the invention is that a length of a lignocellulosic materialis modified, which then permits the length of lignocellulosic materialto be powder coated.

Lignocellulosic material refers to any plant material emanating from thephotosynthetic phenomenon. This includes paper, linen, cotton cloth,woven hessian, and the like.

Thus, the length of a lignocellulosic material may be for example alength of paper, a length of a composite lignocellulosic material, e.gchip board or fibre board, or a length of timber e.g a peeled, sliced orsawn thin section of timber.

A method of impregnating the length of a lignocellulosic material withan impregnating composition, and the nature of various components of theimpregnating composition itself, are fully described in South AfricanPatent Application No. 97/1161, now South African Patent No. 97/1161published on Oct. 29, 1997. (corresponding to PCT/GB 97/00440) which isnow WO97/45591 published Dec. 14, 1997 incorporated herein by reference.Nevertheless, certain details of this impregnating composition and themethod are set out below.

The suitable non-aqueous solvent for the anhydride and the suitablenon-aqueous solvent for the isocyanate resin may be the same or may bedifferent but compatible.

The dicarboxylic anhydride may be selected from the group consisting ofmaleic anhydride, phthalic anhydride, succinic anhydride andtetrahydrophthalic anhydride, and the tricarboxylic anhydride may betrimellitic anhydride. Suitable solvents include methyl acetate, ethylacetate, methylethyl ketone, benzene, trichloroethylene anddichloromethane, preferably dichloromethane Another suitable solvent isliquid carbon dioxide.

The choice of solvent is dictated by its suitability including toxicity,ease of handling, boiling point and evaporative rate, which in turnaffect its ease of recovery from the lignocellulosic material afterimpregnation, its inertness and therefore absence of interferencechemically, flammability and danger of explosion, its solvency therebypropagating the infusion and intimate wetting of the cellular tissue ofthe lignocellulosic material, and finally its ease of recovery, e.g. byabsorption in activated carbon followed by steam purging anddistillation, or condensation and refrigeration or membrane or sievetechnologies or optionally, in the case of liquid carbon dioxide,allowing escape to the atmosphere. Examples of suitable solvents aremethyl acetate, ethyl acetate, methylethyl ketone, benzene,trichloroethylene and dichloromethane. Dichloromethane is the preferredsolvent, because it is non flammable, has a boiling point ofapproximately 39° Centigrade and is relatively inert, and meets theother requirements of the process. In addition dichloromethane has thepropensity to absorb water as a solute forming a 98% azeotrope therebydenaturing the lignocellulosic material and further propagating thelatency of he isocyanates which react with hydroxyl containingcompounds, notably water, to produce urethanes. The high evaporativerate of dichloromethane also propagates the more rapid evaporation ofresidual water.

Another suitable solvent is liquid carbon dioxide.

Liquid-carbon dioxide is a supercritical fluid solvent maintained inprocessing at a temperature of the order of -40° C., and a pressure of18 atmospheres.

It is often a waste product tom other processes, is non-polluting, isinexpensive, and meets the other requirements of the non-aqueoussolvent.

In order to remove the carbon dioxide solvent from the lignocellulosicmaterial pressure is gradually released after the removal of the excessimpregnating composition, and he carbon dioxide is released to theatmosphere, or recaptured for reuse.

When the solvent is removed, the residual carboxylic acid groups have adielectric loss factor such that the modified lignocellulosic materialis able to conduct electricity, thereby sustaining an electrostaticfield allowing the length of lignocellulosic material to beelectrostatically powder coated.

The reaction between the anhydride and the lignocellulosic material atelevated temperatures in the absence of solvents is an esterificationreaction yielding, as an example, lignocellulosic maleate or phthalateor succinate with a residue of water. The anhydrides may be representedas follows: ##STR1##

Other anhydrides such as propionic and butyric anhydride may beesterified to wood or other lignocellulosic material. The result of thereaction is effectively a lignocellulosic polyester, because in thecases of maleic anhydride, phthalic anhydride and succinic anhydride, apolymerisation takes place resulting in binding properties when theimpregnated and dried material is subjected to heat and pressure,thereby complimenting the function of the resin used in this invention.In the case of maleic anhydride, the double bond opens allowing crosslinking and in the case of phthalic anhydride, the ring opens initially,followed by polymerisation.

A further notable function of the anhydrides is that they scavenge anyavailable hydroxyl groups or water, thereby further promoting thelatency of the isocyanates in the impregnating liquor (when present) bypreventing the reaction of these isocyanates with hydroxyl groups whichwould give rise to the formation of urethane polymers, and alsodenaturing the lignocellulosic material during the impregnation process.

A still further function of the anhydrides is that after contact withthe lignocellulosic material and the removal of the solvent the residualcarboxylic acid groups catalyst the polymerisation of the isocyanates.

The impregnating composition may also include a long chain carboxylicacid such as a C10 to C50 monocarboxylic acid, preferably stearic acid,dissolved in a suitable solvent, such as methyl acetate, ethyl acetate,methylethyl ketone, benzene, trichloroethylene and dichloromethane.

A number of carboxylic acids may be esterified to wood or otherlignocellulosic materials in the absence of solvents at elevatedtemperatures. Apart from the esterification potential, the long chaincarboxylic acids with a relatively small polar group attached, tend toorientate with the polar group to the hydroxyl groups in the polymers ofthe lignocellulosic cell walls, with the long carbon chain orientatedtoward water ingress, thereby imposing hydrophobicity.

The impregnating composition preferably contains from 0.25% to 30%inclusive, more preferably from 0.25 % to 15% inclusive of the anhydrideby weight of the impregnating composition.

As the lignocellulosic material preferably takes up from 50% to 150%inclusive, more preferably from 90% to 110% inclusive of theimpregnating composition by weight of the lignocellulosic materialbefore removal of the solvent, after removal of the solvent the mount ofthe anhydride in the lignocellulosic material thus ranges from 0.125% to45% inclusive by weight of the lignocellulosic material, more usuallyfrom 2% to 12% inclusive by weight of The lignocellulosic material.

The impregnating composition may include an isocyanate thermosettingresin dissolved in a suitable non-aqueous solvent. The solvent for theisocyanate resin is preferably the same as the solvent for theanhydride, which is preferably dichloromethane or liquid carbon dioxide,but may be a different compatible solvent.

Isocyanates are compounds containing The group--N═C═O and arecharacterised by the general formula:

    R(NCO).sub.x

wherein x is variable and denotes the number of NCO groups, and Rdenotes a suitable group.

Examples of organic isocyanates include aromatic isocyanates such as m-and p-phenylenediisocyanate, toluene-2,4- and 2,6-diisocyanates,diphenylmethane-4,4'diisocyanate, diphenylmethane-2,4-diisocyanate,chlorophenylene-2,4-diisocyanate, diphenylene-4,4'-diisocyanate,4,4'diisocyanate-3,3'dimethyidiphenyl,3-methyldiphenylmethane4,4'-diisocyanate and diphenyletherdiisocyanateand 2,4,6-triisocyanatotoluene and 2,4,4'-triisocyanatodiphenylether.There may be present mixtures of isocyanates for example a mixture oftoluene diisocyanate isomer such as the commercially available mixturesof 2,4 and 2,6-isomers and also the mixture of di and higherpolyisocyates produced by phosgenation of aniline/formaldehydecondensates. Such mixtures are well-known in the art and include thecrude phosgenation products containing mixtures of methylene bridgedpolyphenylpolyisocyanates including diisocyanate, triisocyanate andhigher polyisocyanates together with any phosgenation by-products.

Preferred compositions are those wherein the isocyanate is an aromaticdiisocyanate or polyisocyanate of higher functionality in particularcrude mixtures of methylene bridged polyphenylpolyiscyanates containingdiisocyanate, triisocyanate and higher functionality polyisocyanates.The methylene bridged polyphenylpolyisocyanates are well-known in theart and are sometimes referred to as polymeric methylene bridgedpolyphenyldiisocyanate (MDI) having an isocyanate functionality rangingfrom 2,5-3 and other products sometimes referred to as crude MDI havinghigher functionality. They are prepared by phosgenation of correspondingmixtures of polyamines obtained by condensation of aniline andformaldehyde.

Specific examples of suitable isocyanates are those having an (NCO)content percentage preferably exceeding 20%, more preferably exceeding25%. These isocyanates promote latency or reduced reactivity because ofthe high number of NCO groups, and provide the maximum capacity forhydroxyl bonding. Examples are Desmadur VKS or Desmadur VK by Bayer,which are solvent free mixtures of aromatic polyisocyanates such asdiphenyl methane-4,4 di-isocyanate and polymeric matter. These andsimilar are among those referred to as MDIs in the industry. A furtherdescription used is a di-isocyanate-diphenyl methane, further examplesbeing Suprasec DNR-5005, which is a polymeric MDI, or Suprasec 2020which is a monomeric MDI with available NCO percentages of 30.7% and 29%and which are polymeric MDI with standard functionality and monomericMDI respectively. The Suprasec resins are supplied by ICI. A fartherexample of a crude MDI is Voronate M 229 by Dow Chemical Company.

Further suitable di-isocyanates are the Toluene di-isocyanates with thealternative names tolylene di-isocyanate or toluylene di-isocyanate withthe abbreviation TDI, such as Desmadur L75 by Bayer.

A further example of the principle of wood esterification is the use ofethyl isocyanate which reacts with hydroxyl groups to form ethylcarbamate (urethane) according to the formula:

    C.sub.2 H.sub.5 NCO+H.sub.2 O→NH.sub.2 COOC.sub.2 H.sub.5

The isocyanate resins are folly soluble in dichloromethane and reactwith the hydroxyl groups on the cellulose and hemi cellulose moleculesof the lignocellulosic material to form a wood ester, In this way theyform a chemical bond adhesion rather than a cohesive adhesion. They aretherefore effective in contributing not only to a reduction in watersensitivity but also to superior binding. In addition, they scavenge anycarboxyl groups which are residual from the carboxylic acid derived fromthe anhydride. The isocyanate resins lend themselves to synergisticbinding of composites and to the propagation of superior mechanicalproperties by a two way linkage with the residue of the anhydrides andthe hydroxyl groups on the lignocellulosic material itself.

The impregnating composition preferably contains the isocyanatethermosetting resin in an amount of from 1.5% to 60% inclusive of theisocyanate thermosetting resin by weight of the impregnatingcomposition.

The impregnating composition preferably includes both an anhydride andan isocyanate resin, for the best results.

In the impregnating composition there may also be incorporated otheradditives such as for trample a fire retardant or fire inhibitor, abacteriostat, a fungicide, an insecticide, an ultraviolet light absorberor stabiliser, an anti oxidant, a hydrophobic agent such as a siliconeor siloxane, or a wax.

The impregnation is preferably conducted by irrigating the movinglengths in a reel to reel configuration, or in a reel to flatconfiguration. The impregnating composition immediately wets the paperthroughout its depth, and the weight of the impregnating compositionapplied per unit area of the paper is accurately controlled.

Alternatively, when the lignocellulosic material is paper, the paper maybe wound into loose rolls of from 200 mm to 1400 mm in width anddiameters of up to 11/2 meters, may be impregnated by placing them in animpregnation cylinder or autoclave. The cylinder is then sealed andsubjected to a vacuum. This exhausts all air from the paper and frombetween the windings in The roles. The vacuum line is isolated and theimpregnating composition is cascaded into the cylinder until full.Pressure is now exerted either hydraulically or pneumatically to ensurethorough impregnation uniformly throughout the mass of the material. Thecylinder is drained and the charge is subjected to a post vacuum inorder to remove all excess impregnating composition which is alsoreturned to its receptacle. The charge is now subjected to induced heatin order rapidly to evaporate the solvent. The heal induction may be bybearing coils around the cylinder or alternatively by the introductionof hot air circulating around the charge serving both to convey heat andto tarry the rapidly evaporating solvent, or by microwave or by anycombination. The solvent laden air passes from the cylinder, overcondensation coils onto which the solvent condenses and thence againpast the heating elements, and back into the cylinder on a closed loop.Mechanical compression may also be used to further facilitatecondensation. As the process of the recovery of solvent nearscompletion, the residual air is then passed through activated carbon orthrough a membrane in order to "polish" the emitted air to conform toemission standards.

As is indicated above, after the paper has been impregnated with theimpregnating composition, there is removed from the impregnated paperany excess of the impregnating composition and then there is removed thenon-aqueous solvent or solvents, preferably for reuse.

When the length of a lignocellulosic material is for example a length ofwood or wood veneer or chipboard or the like, the impregnation may beconducted by placing the length of lignocellulosic material in asuitable container such as a pressure cylinder, and introducing theimpregnating composition into the container, impregnating the length oflignocellulosic material by any of the cycles: vacuum/pressure/vacuum,or vacuum/vacuum, or pressure/greater pressure/vacuum; removing the fromthe impregnating composition from the container; and removing thesolvent from the impregnated length of lignocellulosic material.

In step (b) of the method, there is removed from the impregnated lengthof lignocellulosic material any excess of the impregnating composition.This step is obviously only necessary where there is excess of theimpregnating composition in the length of lignocellulosic material.

In step (c) of the method, there is removed from the impregnated lengthof lignocellulosic material the non-aqueous solvent or solvents. Thismay be achieved using electronically induced heat such as infra redinduced heat. The solvents are preferably recaptured for reuse.

Prior to step (d) of the method, if it is desired to form a laminate oftwo or more lengths of lignocellulosic material impregnated as describedabove, an adhesive may be applied between each sheet and the sheets maythen be laminated together either in a flat or corrugated configuration,with hear to cause the adhesive to set.

In step (d) of the method, the impregnated length of lignocellulosicmaterial is placed in an electrostatic field or in a fluidized bed and apowder coating composition is applied thereto.

Generally, the powder coating composition, in the form of a finelydivided pre-formulated dry powder, is propelled towards the surface oflignocellulosic material from a suitably charged applicator gun, eitherfriction or electrostatic, such that the particles of the powder coatingcomposition adhere to the surface of the length of lignocellulosicmaterial. Electrostatic charged guns are preferred such as the SUPERCARONA by Gema. Any particles of the powder coating composition that donot adhere to the surface of the length of lignocellulosic material,fall from the length of lignocellulosic material and may be recovered.

Examples of suitable powders are polyurethanes or epoxy polyesters forinterior use or pure polyesters for exterior use, in gloss, suede ormatt, in textures, hammer tones, metallics, pearlescents, wrinklefinishes or multi colours. Curing temperatures are from as low as 100°C. in the presence of ultra violet light using photosensitive catalysis,or in the range of 140°-185° C., with cure times of a few seconds tothree minutes.

In step (e) of the method, the length of lignocellulosic material issubjected to elevated temperatures to polymerise and/or cross-link theresin or resins in the impregnated length of lignocellulosic materialand to cure the powder coating composition to form the powder coating.

For example, the length of lignocellulosic material may be passedthrough a space beater in which the temperature of the length oflignocellulosic material is raised to a level above 140° C., moreusually above 185° C.

At the conclusion of the beating step, the powder coating composition isfully cured.

The impregnating composition provides the length of lignocellulosicmaterial with improved properties of strength, water resistance, andsurface stability. In addition, the powder coating composition maycross-link with available NCO groups from the impregnating resin,resulting in a chemical adhesion of the powder coating to the length oflignocellulosic material.

It is the anhydride or isocyanate resin, in the suitable non-aqueoussolvent, in the impregnating composition which provides thelignocellulosic material with the required dielectric properties. Forexample maleic anhydride in dichloromethane has a dielectric loss factorof 0.97 from which it may be deduced that it has the capacitativeproperties to allow the acceptance of electric charge and allow thegrounding of the lignocellulosic material in the electrostatic field. Bycomparison dichloromethane on its own has a dielectric loss factor of0.25 and a 10% solution of an isocyanate in dichloromethane has adielectric loss factor of 0.26.

The dielectric constants of various materials for use in the inventionare set out below:

    ______________________________________                                        f(MHz)   ε'    ε"                                                                            tan δ                                    ______________________________________                                        PTFE rod-Control                                                               651     2.00          <0.001  0.0005                                         1502     2.00          <0.001  0.0005                                         2356     2.01          0.001   0.0005                                         3208     2.02          0.002   0.0010                                         Maleic anhydride dry powder                                                    651     2.34          <0.002  <0.0008                                        1504     2.31          <0.002  <0.0008                                        2359     2.32          <0.002  <0.0008                                        3214     2.33          <0.002  <0.0008                                        Sample 2020 Suprasec by ICI (isocyanate resin)                                 651     3.87          0.568   0.1470                                         1503     3.61          0.394   0.1092                                         2357     3.58          0.312   0.0822                                         3211     3.60          0.312   0.0867                                         Sample 103 Suprasec by ICI (flexible isocyanate resin)                         651     3.44          0.365   0.1063                                         1503     3.27          0.284   0.0869                                         2357     3.21          0.254   0.0790                                         3211     3.21          0.255   0.0795                                         Sample 5005 Suprasec by ICI                                                    651     3.65          0.404   0.1109                                         1503     3.47          0.274   0.0789                                         2357     3.46          0.233   0.0675                                         3210     3.47          0.227   0.0654                                         ______________________________________                                    

The reference measurement of PTFE yielded values of ε' and ε" within themeasurement tolerance of the equipment (i.e., -5% on ε').

Maleic anhydride powder is almost totally lossless and would not heat ina microwave field.

Samples 2020, 103 and 5005 (isocyanate resins) are quite similar andwould all heat substantially in a microwave oven.

Examples of suitable lengths of lignocellulosic material to be treatedby the method of the invention include lengths of paper having a weightof 125 g, 160 g, 230 g, 340 g, 450 g or 560 g per m², or multi laminatesof sheets of paper in flat or shaped form. Other suitable materialsinclude lengths of wood or wood veneer, or chipboard or the like.

When the length of lignocellulosic material is a sheet of paper, afterthe powder coating, the powder coated sheet of paper may be attached toanother substrate such as for example chipboard, medium densityfibreboard, cement fibre board, cement bonded particle board, orplywood, to provide such products with decorative surface.

For example, a powder coated sheet of paper may be applied to asubstrate with an adhesive, typically in low pressure presses such asveneer presses or continuous laminating plants.

The method of the invention has the main advantage that it allows apowder coating composition to be applied to articles which previouslyhave not been able to be powder coated. The modification of a length ofa lignocellulosic material provides the length of lignocellulosicmaterial with the required dielectric properties to allow a powdercoating to be applied thereto. In particular, the method of theinvention allows a powder coating composition to be applied a sheet ofpaper. The paper so coated may then be applied to another substrate.This has advantages including cost advantages, and ease of working andthe like.

What is claimed is:
 1. A method of applying a powder coating to a lengthof a lignocellulosic material includes the steps of:(a) impregnating thelength of lignocellulosic material with an impregnating compositioncomprising either;(i) a dicarboxylic anhydride or a tricarboxylicanhydride dissolved in a suitable non-aqueous solvent; or (ii) anisocyanate thermosetting resin dissolved in a suitable non-aqueoussolvent; or (iii) a combination of a dicarboxylic anhydride eor atricarboxylic anhydride and an isocyanate thermo-setting resin dissolvedin a suitable non-aqueous solvent; (b) if necessary removing from theimpregnated length of lignocellullosic material any excess of theimpregnating composition; (c) removing the non-aqueous solvent; (d)placing the impregnated length of lignocellulosic material in anelectrostatic field or in a fluidized bed and applying a powder coatingcomposition thereto so that the powder coating composition adheresthereto; and (e) then subjecting the length of impregnated and coatedlignocellulosic material to elevated temperatures to polymerise and/orcross-link the resin of step (a) in the length of lignocellulosicmaterial and to cure the powder coating composition to form a powdercoating.
 2. A method according to claim 1 wherein the length oflignocellulosic material is selected from the group consisting of asheet of paper, a length of peeled or sliced wood veneer, a length oflaminated wood, and a length of chip board.
 3. A method according toclaim 1 or claim 2 wherein the impregnating composition comprises:(iii)a combination of a dicarboxylic anhydride or a tricarboxylic anhydrideand an isocyanate thermosetting resin dissolved in a suitablenon-aqueous solvent.
 4. A method according to claim 1 wherein thedicarboxylic anhydride is selected from the group consisting of maleicanhydride, phthalic anhydride, succinic anhydride and tetrahydrophthalicanhydride, and the tricarboxylic anhydride is trimellitic anhydride. 5.A method according to claim 1 wherein the suitable non-aqueous solventfor the anhydride and the suitable non-aqueous solvent for theisocyanate thermosetting resin art selected from the group consisting ofmethyl acetate, ethyl acetate, methylethyl ketone, benzene,trichloroethylene and dichloromethane.
 6. A method according to claim 5wherein the solvent is dichloromethane.
 7. A method according to claim 1wherein the suitable non-aqueous solvent for the anhydride and/or thesuitable non-aqueous solvent for the isocyanate thermosetting resin isliquid carbon dioxide.
 8. A method according to claim 1 wherein theimpregnating composition contains from 0.25% to 30% inclusive of theanhydride by weight of the impregnating composition.
 9. A methodaccording to claim 1 wherein the impregnating composition contains theisocyanate thermosetting resin in an amount of 1.5% to 60% inclusive ofthe isocyanate thermosetting resin by weight of the impregnatingcomposition.
 10. A method according claim 1 wherein the powder coatingcomposition is selected from the group consisting of polyurethanes,epoxy polyesters and polyesters.
 11. A method according to claim 1wherein in step (e) the length of lignocellulosic material is passedthrough a space heater in which the temperature of the length oflignocellulosic material is raised to a level above 140° C.
 12. A methodaccording to claim 11 wherein the temperature of the length oflignocellulosic material is raised to a level above 185° C.
 13. A methodaccording to claim 1 wherein in step (e) the length of lignocellulosicmaterial is passed through a space heater in the presence ofultra-violet light.